421 4.7.0: Temporarily deferred, often for low reputation
421 4.7.0 is a temporary refusal on security or policy grounds. For cold email it most often means the receiver does not trust your sending domain or IP yet and asks you to slow down. It is the first warning, not the last.
- Bounce type
- Soft bounce (temporary)
- Usually caused on
- Your sending side
- Terlido report label
- Blocked by the provider
What the standard says
RFC 5321 defines 421 as "service not available, closing transmission channel": the server is shutting the connection and wants you to try again later. RFC 3463 defines X.7.0 as "Other or undefined security status": a security or policy reason applies, but no more exact code was chosen. Class 4 means the refusal is temporary.
What the mailbox providers say
- Gmail (421 4.7.0): “This message is suspicious due to the very low reputation of the sending IP address.” Source
- Gmail (421 4.7.0): “This message is suspicious due to the very low reputation of the sending domain.” Source
- Gmail (421 4.7.0): “TLS required for RCPT domain, closing connection.” Source
- Gmail (421 4.7.0): “Try again later, closing connection.” Source
- Yahoo (421 or 451): “A 421 or 451 SMTP error indicates a temporary problem blocking the delivery of your message.” Source
- Yahoo (TS*): “This is a temporary error and your mail server may automatically re-try sending the email at a later time.” Source
Common causes
- A new domain or a new mailbox that has no sending history and has been pushed to full volume too fast.
- A reputation that dropped because of spam complaints or a high rate of bounces.
- A sudden jump in the number of messages per day from the same sender.
- Missing or weak authentication, so the receiver cannot tie your mail to your domain.
- On the TLS variant, a sending server that does not offer encryption where the receiver demands it.
- A sending IP shared with other customers, where another sender on the same IP damaged its reputation.
How to fix it
- Slow down. Cut the daily volume for the affected mailbox and let it recover before you raise it again.
- Warm up new mailboxes and domains and increase the volume in small steps.
- Check SPF, DKIM and DMARC for the sending domain and fix every failure.
- Look at your complaint and bounce rates and remove contacts that bounce or complain.
- Enable TLS on your own sending server if the text mentions it.
- Compare the deferral rate between providers. If only one provider defers, fix that provider's issue first.
- Pause the least engaged part of the audience and send to people who are most likely to read your message.
Is it a deliverability problem?
Yes, this code is the main early warning that a sender-side deliverability problem is building. The messages are only deferred now. If you ignore the signal and keep the volume up, the same receivers move on to permanent blocks such as 5.7.1. Act on the first deferrals. Two details matter here. The first is the speed of your reaction, because deferrals are the stage where cheap fixes still work. The second is that a deferred message may still go out: Yahoo says your mail server may retry a temporary error later. A retry does not cure a reputation problem, so do not rely on it.
How Terlido handles it
When a bounce message for this code arrives in a connected mailbox, Terlido reads the status code. An enhanced code that starts with 4 is a soft bounce. In the campaign Reports tab this one is grouped as "Blocked by the provider".
On a soft bounce Terlido logs the event on the lead's activity. The lead is not stopped, and the sequence continues at its next scheduled step.
Check the domain behind the bounce
The free domain checker runs live SPF, DKIM, DMARC, MX and blocklist lookups and explains each result in plain English.
Free tools that help
- Email blocklist checkerCheck a bounded set of named public blocklists.
- Email warmup schedule calculatorSee Terlido's warmup ramp day by day: 5 messages on day one, about 10% more each day, never more than 3 extra, up to your target.
Related error codes
- 421 4.7.28: Unusual rate of unsolicited mail from your domain
- 421 4.7.26: Rate limited because mail is unauthenticated
- 550 5.7.1: Delivery not authorized, message refused
- 451 4.3.0: Temporary mail system problem